Up to the present time, this method has been implemented by adding a sensor to the pre-existing tire monitoring system. An example of this type of implementation is disclosed in patent document EP 2 090 862, which describes a method comprising the use of a magnetic sensor to determine the length of the footprint. This magnetic sensor, placed on the rim, analyzes the magnetic field that it receives. When this magnetic sensor is in the angular portion of the wheel which is in contact with the ground, the magnetic field is modified, enabling the footprint to be calculated on the basis of the magnetic field variation, the dimensions of the wheel and its rotation.
An improvement to this solution is disclosed in patent document FR 2 944 231. This document proposes a solution which is less costly than the use of pressure sensors. It proposes the placing of the magnetometers in a fixed manner, for example on the coil of a shock absorber spring. Each magnetometer measures the magnetic field of a metal belt placed under the tread of the tire near the magnetometer. This magnetic field is modified when the tire is flattened on the ground while rolling. The curve representing the received magnetic field as a function of the angle of rotation of the wheel then exhibits variations directly related to the deformation of the tire in contact with the ground.
These two examples show that the detection of a tire footprint is carried out at present with supplementary devices which are not incorporated into tire management systems such as TPMS or information systems such as TIS (“Tire Information System” in the English terminology). In the interests of cost reduction and simplification of equipment, therefore, it is advantageous to have a method which requires no supplementary device to provide a knowledge of tire footprints.